Human natural killer (NK) lymphocytes with cytotoxicity to human tumor cells are present in the blood of both normal and diseased individuals. Freshly isolated lymphocytes have a latency period of approximately three hours before they can effect cytolysis of most tumor cell lines, whereas cultured lymphocytes are activated and immediately lyse these tumor cell lines. Synthetic, plant and plasma proteinase inhibitors inhibit this natural cell mediated cytotoxicity (Hudig, et al., manuscript in preparation). Inhibition may occur by preventing the conversion of previously inactive lymphocytes into cells capable of immediate cytotoxicity and/or by inactivating the proteinases that are mediators of cytotoxicity. The activated proteinases of NK cells are likely to be associated with the plasma membranes. The activity and role of membrane-bound proteinases will be assessed by determining the concentrations and specificities of inhibitors needed to depress NK activity, measurement of alterations in proteolytic capacities of whole cells, and isolation and characterization of membrane-associated proteinases. Regulation of NK activity by plasma antiproteinases and acute phase reactants will be evaluated, with particular emphasis on alpha 1 antitrypsin, the major serum antiproteinase. This study will contribute to understanding of the mechanism of cell-mediated cytotoxicity and of the effects of the cellular environment of nascent or progressing neoplasia upon cells cytotoxic to tumors.